1. Field of the Invention
This invention relates to a bleaching composition for processing silver halide color photographic materials. More particularly, the invention relates to preventing the generation of toxic gases, such as a halogen gas, from the bleaching solution containing a bleaching agent and a halide.
2. Description of the Prior Art
In the photographic processing of silver halide color photographic materials, photosensitive materials (including silver halide photographic emulsions that may have been previously fogged) are, after being imagewise exposed, usually developed by a developer containing an aromatic primary amine color developing agent in the presence of dye-forming couplers, and then the developed silver formed simultaneously is re-halogenated in a bleaching step and removed by fixing or blixing.
It has hitherto been known to use a prussiate (ferricyanate), a dichromate, a ferric salt, an organic acid ferric complex salt, or a persulfate as a bleaching agent, and a water-soluble halide such as sodium chloride, ammonium chloride, sodium bromide, etc., as a re-halogenating agent in bleaching compositions for the bleaching step.
Prussiates (ferricyanates) are good bleaching agent in that they provide sufficiently high bleaching rate (oxidation rate) and oxidize sufficiently developed within a predetermined period of time. However, a bleaching solution using a prussiate (ferricyanate) as a bleaching agent releases cyanide ions due to photolysis, which causes environmental pollution. Hence the waste bleaching solution must be subjected to a treatment in order to make the waste solution completely harmless.
A bleaching solution using a dichromate as a bleaching agent results in formation of dichromate ions, which also causes problems, and hence the waste solution thereof must also be subjected to a treatment for making it completely harmless.
These conventional compositions which require an extra treatment for making the waste solutions thereof completely harmless have the defect of imposing serious burden on their industrial applications.
It has also been known to use solutions-containing persulfates, inorganic ferric salts, organic acid ferric complexes, etc., as a bleaching solutions, in order to overcome the difficulties encountered using prussiates (ferricyanates) or dichromates. Some of these bleaching agents, especially inorganic ferric salts, are insoluble in weakly acidic or alkaline solutions, and the bleaching rate is insufficiently high under such conditions. Hence the bleaching solution is frequently used at a low pH, in order to increase the oxidizing power, and thereby the bleaching rate is increased. Increasing the oxidizing power by reducing the pH of the bleaching solution can also be effective when using dichromates, but is particularly effective for practical use when using persulfates or ferric salts, in that the burden required for the treatment of the waste solution is slight.
However, when the oxidizing power of a bleaching solution using a persulfate or an inorganic ferric salt as the bleaching agent is increased by reducing the pH of the bleaching solution, the bleaching agent reacts with chloride or bromide existing in the bleaching solution as a re-halogenating agent to generate chlorine or bromine, respectively which causes serious problems in the working environment. Furthermore, the formation of chlorine or bromine is accompanied by the corrosion of parts of the processing equipment. Therefore, it is very difficult to use bleaching processes using a persulfate or an inorganic ferric salt as the bleaching agent unless the above-mentioned difficulties are overcome.
Various substances supposedly capable of preventing the formation of chlorine or bromine by the oxidation of a chloride or a bromide with a bleaching agent contained in a bleaching solution have been proposed, such as gelatin, various amino acids, aliphatic monocarboxylic acids, cobalt salts, etc., as described in Research Disclosure, 17556 (1978, November). However, many of these compounds have the defect that they do not always show a sufficient effect to prevent the generation of chlorine or bromine. Even if they have an excellent prevention effect, they may give off an irritating smell or cause rust, or suffer the defect that they are lacking in stability in a processing solution and accelerate the decomposition of the persulfate, thereby reducing the effective life of the bleaching solution and the bleaching power thereof. Others suffer the defect that the pH of a bleaching solution containing such compounds deviates due to their presence, and lacks in stability. Thus a need for improved bleaching solution is apparent.